How to edit a "GPL version 2" code and provide it to the customer (to get paid)?

Question

A customer (from a freelancer website, who awarded me this single project) has asked me to add some extra functionality to a Wordpress plugin which is licensed under "GPL version 2", the customer explicitly mentioned this plugin's name and asked me to either add the extra functionality to it, or code the whole thing from scratch.

I decided to edit the plugin since it's allowed under GPL v2 (I think, please let me know if I'm wrong) but I'm not sure how I should provide it to customer after it's edited...

I mean, based on my understanding after reading GPL v2 , I should include something like the following text above all the files that I edit, right?

This file was modified on Jan 12, 2020 to add extra functionality to the original XYZ plugin

And then I provide it with the same name as the original plugin to customer? nothing else required?

Answer 1

There usually is a simple way to do so with open source projects:

Create a clone of the repository of the project. If it doesn't exist, create a repository and import the version you start with into it.

Make the modifications you want and commit them with meaningful commit messages. Make an amendment to the readme that states your contribution. That can be as little as adding your name to the copyright list like[1]

§10 Attributions / Copyright
(c) 2004-2018 someone else
(c) 2019 based on version x.Y, Addition of feature XY by me

You can be even more verbose in the title and name it like Projectname-myname. The by-line changes in the source files are documented by your usage of the VCS.

When done, push the repository with your changes to some repository accessible to your customer. Give this version to your customer.

If you're a nice guy and your contract allows and if the upstream project of that plugin is alive and kicking, consider to create a pull request for your changes and work with them to get your changes integrated (the maintainers will tell you how they want to receive them and in which format). You might do this right from the start for active projects.

[1]Check your contract (and your local laws if the contract doesn't specify it) who owns the copyright on your work. If your customer owns it, put their name in the copyright line.